Well treating composition and method

ABSTRACT

There is disclosed a method of treating a subterranean substantially carbonate-free sandstone formation associated with a well completed in said substantially carbonate-free sandstone formation comprising displacing into said formation through said well an effective amount of an acidic aqueous solution comprising from about 5 to about 36 weight % phosphoric acid, from about 0.6 to about 5 weight % of a highly polar surface active agent which is a citric acid-monoethanolamine elimination reaction product, and an effective amount of a ferrous metal corrosion inhibitor. For treating a subterranean limestone or dolomitic formation or a sandstone formation having a substantial carbonate content there is additionally included in the acidic aqueous solution an amount of a phosphate crystal modifying agent sufficient to control crystal growth, minimize crystal agglomeration and minimize crystal adhesion to the formation. The acidic aqueous solutions exhibit clay shrinking and/or stabilizing properties and a lengthy spending time relative to that of conventional hydrochloric acid-based acidizing compositions.

This application is a division, of application Ser. No. 97,151, filed11/26/79, now abandoned.

FIELD OF THE INVENTION

This invention relates to workover and completions fluids for use inwells completed in subterranean formations and to methods of treatingassociated subterranean formations and perforating cased wells.

DESCRIPTION OF THE PRIOR ART

Acidic aqueous solutions are frequently used to treat subterraneanformations associated with a well to increase the permeability of theformation so as to be more productive or, in the case of injectionwells, more receptive to fluid displacement into the formation.

Hydrochloric acid is commonly used in acidizing fluids, although anumber of other acids such as hydrofluoric acid, phosphoric acid, aceticacid, formic acid, among others, and acid mixtures, have been employed,normally with a variety of additives such as surfactants, demulsifiers,corrosion inhibitors, etc.

Dependent upon the formation, acidizing fluids are commonly employed todissolve carbonates or other soluble organic deposits and/or todehydrate, or shrink, clays.

A problem common to acidizing is the production of precipitates withinthe formation pores by the action of the acid or its byproducts on someprecipitate-forming constituent of the formation. The precipitates,which are generally produced when the acid is spent, can plug theformation. This can be a particular problem with acidizing fluidsemploying phosphoric acid, or an acid mixture containing phosphoricacid, since calcium and magnesium compounds which are commonly found insome formations form insoluble phosphates when contacted with phosphoricacid.

Another common problem in clay-containing formations is that clays whichare dehydrated by an acidizing fluid, unless somehow stabilizied, willsubsequently rehydrate in the presence of spent acidizing fluid orformation water. In some cases the clays swell to a degree such thatvolumetrically they occupy more space than that prior to theacidization. In some cases there is also a tendency for the dehydratedclays to disperse within the formation and then subsequently rehydratein the presence of spent acidizing fluids or formation water. In eithercase the result can be a significant lessening of formation permeabilityrelative to that prior to the acidization.

An additional drawback relatively common to the prior art is that manyacidizing fluids must be handled with extreme care in that they employfuming acids, and are caustic to skin. Furthermore, environmentally safedisposal of prior art acidizing fluids, even if spent, can be a problem.

The present invention is concerned with multi-functional acidizingfluids in the form of specified acidic aqueous solutions which are freefrom a number of the disadvantages associated with prior art acidizingfluids, with methods of treatment of subterranean formations withspecified acidic aqueous solutions and with methods of perforating casedwells using specified acidic aqueous solutions as perforating fluids.

Broadly the invention provides a workover and completions fluid for usein wells completed in limestone or dolomitic formations or in sandstoneformations having a substantial carbonate content, which comprises anacidic aqueous solution comprising phosphoric acid, a citricacid-monoethanolamine elimination reaction product, a ferrous metalcorrosion inhibitor and a phosphate crystal modifying agent, and methodsof displacing such an acidic aqueous solution into limestone ordolomitic formations or sandstone formations having a substantialcarbonate content.

The invention also provides methods of treating substantiallycarbonate-free sandstone formations by displacing into the formation anacidic aqueous solution comprising phosphoric acid, citricacid-monoethanolamine elimination reaction product and a ferrous metalcorrosion inhibitor, as well as methods of perforating cased wellsemploying the acidic aqueous solutions as the perforating fluids.

The citric acid-monoethanolamine elimination reaction product employedin the acidic aqueous solutions is essentially that taught in SchwartzU.S. Pat. No. 3,095,379, issued June 25, 1963, as will be discussed ingreater detail hereinafter.

While it is clear that Schwartz recognized the compatability of thecitric acid-monoethanolamine reaction product with dilute acidic aqueoussolutions, including phosphoric acid-based solutions, it had notpreviously been recognized, nor expected, that certain acidic aqueoussolutions comprising phosphoric acid and the citricacid-monoethanolamine reaction product possessed a unique ability toshrink and stabilize clays. Furthermore, while Schwartz states at Column1, lines 40 and 41 that the citric acid-monoethanolamine reactionproduct "has outstanding properties of corrosion inhibition" the degreeof corrosion inhibition displayed towards ferrous metal proved to betotally unacceptable for use in wells, at least at bottomholetemperatures. Thus despite the "outstanding properties" described bySchwartz it has been found absolutely essential that the acidic aqueoussolutions additionally comprise an effective amount of a ferrous metalcorrosion inhibitor. While Schwartz also states at Column 3, lines 8 and9, with reference to the citric acid-monoethanolamine reaction product,that "there is no upper limit to the concentration which is usable" ithas been found that for the purposes intended by the present Applicantthat there is an effective upper limit for the product of about 5 weight% as otherwise the aqueous acidic solutions do not possess effectiveclay dispersion properties.

For descaling pumps there has in the past been employed an acidicaqueous solution comprising phosphoric acid, the citricacid-monoethanolamine reaction product, sodium hexametaphosphate, Dowfax2Al® and O.B.Hibit®. It had not however been recognized that theinclusion of sodium hexametaphosphate, a constituent sometimes employedin boiler descaling fluids, in such a solution would damage limestone ordolomitic formations or sandstone formations having a substantialcarbonate content. This lack of recognition was not however unexpectedsince the prior usage involved the descaling of pulled pumps. Thepresent Applicant attempted to improve the permeability of a limestoneformation by the displacement into the formation of such an acidicaqueous solution and blocked the formation. It was subsequentlydetermined that the sodium hexametaphosphate was detrimental to thedesired purpose and, indeed, that is was necessary to additionallyinclude in the formulation a phosphate crystal modifying agent toprevent precipitate blockage of such formations.

SUMMARY OF THE INVENTION

In one particular aspect the present invention provides a workover andcompletions fluid for use in a well completed in a limestone ordolimitic formation or a sandstone formation having a substantialcarbonate content comprising an acidic aqueous solution comprising fromabout 5 to about 36 weight % phosphoric acid, from about 0.6 to about 5weight % of a highly polar surface active agent which is a citricacid-monoethanolamine elimination reaction product characterized by avisible absorption spectrum which displays an increasing extinctioncoefficient from the red to the near U.V. with some structure, anabsorption maximum at 450 nm, a plateau from 460 n, to 465 nm, anabsorption maximum at 450 nm and extinction coefficients as follows:

ε₄₇₅ =0.325 l/gm.cm

ε₄₆₂ =0.379 l/gm.cm

ε₄₅₀ =0.509 l/gm.cm,

an effective amount of a ferrous metal corrosion inhibitor, and anamount of a phosphate crystal modifying agent sufficient to controlcrystal growth, minimize crystal agglomeration and minimize crystaladhesion to the formation.

In another particular aspect the present invention provides a workoverand completions fluid concentrate intended for aqueous dilution to yeildan acidic aqueous solution comprising from about 5 to about 36 weight %phosphoric acid, from about 0.6 to about 5 weight % of a highly polarsurface active agent which is a citric acid-monoethanolamine eliminationreaction product characterized by a visible absorption spectrum whichdisplays an increasing extinction coefficient from the red to the nearU.V. with some structure, an absorption maximum at 475 nm, a plateaufrom 460 nm to 465 nm, and absorption maximum at 450 nm and extinctioncoefficients as follows:

ε₄₇₅ =0.325 l/gm.cm

ε₄₆₂ =0.379 l/gm.cm

ε₄₅₀ =0.509 l/gm.cm,

an effective amount of a ferrous metal corrosion inhibitor, and anamount of a phosphate crystal modifying agent sufficient to controlcrystal growth, minimize crystal agglomeration and minimize crystaladhesion to the formation, said concentrate comprising up to about 85weight % phosphoric acid, sufficient of said citricacid-monoethanolamine elimination reaction product to provide upondilution an amount of from about 0.6 to about 5 weight %, an amount ofsaid ferrous metal corrosion inhibitor sufficient upon dilution toinhibit ferrous metal corrosion, and an amount of said crystal modifyingagent sufficient upon dilution to control crystal growth, minimizecrystal agglomeration and minimize crystal adhesion to the formation.

In yet another particular aspect the present invention provides a methodof treating a subterranean formation associated with a well comprisingdisplacing into said formation through said well an effective amount ofan acidic aqueous solution comprising from about 5 to about 36 weight %phosphoric acid, from about 0.6 to about 5 weight % of a highly polarsurface active agent which is a citric acid-monoethanolamine eliminationreaction product characterized by a visible absorption spectrum whichdisplays an increasing extinction coefficient from the red to the nearU.V. with some structure, an absorption maximum at 475 nm, a plateaufrom 460 nm to 465 nm, an absorption maximum at 450 nm and extinctioncoefficients as follows:

ε₄₇₅ =0.325 l/gm.cm

ε₄₆₂ =0.379 l/gm.cm

ε₄₅₀ =0.509 l/gm.cm,

and an effective amount of a ferrous metal corrosion inhibitor.

In a further particular aspect the present invention provides a methodof treating a subterranean substantially carbonate-free sandstoneformation associated with a well completed in said substantiallycarbonate-free sandstone formation comprising displacing into saidformation through said well an effective amount of an acidic aqueoussolution comprising from about 5 to about 36 weight % phosphoric acid,from about 0.6 to about 5 weight % of a highly polar surface activeagent which is a citric acid-monoethanolamine elimination reactionproduct characterized by a visible absorption spectrum which displays anincreasing extinction coefficient from the red to the near U.V. withsome structure, an absorption maximum at 475 nm, a plateau from 460 n,to 465 nm, an absorption maximum at 450 nm, and extinction coefficientsas follows:

ε₄₇₅ =0.325 l/gm.cm

ε₄₆₂ =0.379 l/gm.cm

ε₄₅₀ =0.509 l/gm.cm,

and an effective amount of a ferrous metal corrosion inhibitor.

In yet a further particular aspect the present invention provides amethod of treating a subterranean limestone or dolomitic formation or asandstone formation having a substantial carbonate content associatedwith a well completed in said limestone or dolomitic formation or saidsandstone formation having a substantial carbonate content comprisingdisplacing into said formation through said well an effective amount ofan acidic aqueous solution comprising from about 5 to about 36% byweight phosphoric acid, from about 0.6 to about 5 weight % of a highlypolar surface active agent which is a citric acid-monoethanolamineelimination reaction product characterized by a visible absorptionspectrum which displays an increasing extinction coefficient from thered to the near U.V. with some structure, an absorption maximum at 475nm, a plateau from 460 nm to 465 nm, an absorption maximum at 450 n, andextinction coefficients as follows:

ε₄₇₅ =0.325 l/gm.cm

ε₄₆₂ =0.379 l/gm.cm

ε₄₅₀ =0.509 l/gm.cm,

an effective amount of a ferrous metal corrosion inhibitor, and anamount of a phosphate crystal modifying agent sufficient to controlcrystal growth, minimize crystal agglomeration and minimize crystaladhesion to the formation.

In even a further particular aspect the present invention provides in amethod of perforating a cased well where a perforating fluid isintroduced into the casing to minimize the risk of formation damageresulting from desired or incidental contact of the formation by theperforating fluid upon perforation the improvement comprising utilizingas the perforating fluid an acidic aqueous solution comprising fromabout 5 to about 36% by weight phosphoric acid, from about 0.6 to about5% by weight of a highly polar surface active agent which is a citricacid-monoethanolamine elimination reaction product characterized by avisible absorption spectrum which displays an increasing extinctioncoefficient from the red to the near U.V. with some structure, anabsorption maximum at 475 nm, a plateau from 460 nm to 465 nm, anabsorption maximum at 450 nm and extinction coefficients as follows:

ε₄₇₅ =0.325 l/gm.cm

ε₄₆₂ =0.379 l/gm.cm

ε₄₅₀ =0.509 l/gm.cm,

and an effective amount of a ferrous metal corrosion inhibitor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The acidic aqueous solutions of the invention and the acidic aqueoussolutions employed in the methods of the invention possess clayshrinking and stabilizing properties, an ability to dissolve organicdeposits such as carbonates, and an ability to penetrate and disperseinsoluble deposits such as muds and clays.

The acidic aqueous solutions are non-fuming, and are not caustic toskin. Additionally, the acidic aqueous solutions do not createenvironmental safety disposal problems and, in fact, at properconcentrations can be excellent fertilizers.

The acidic aqueous solutions are also relatively noncorrosive, due tothe use of effective inhibitors. A representative acidic auqeoussolution of the invention did not show a measurable rate of corrosion of1020 mild steel at room temperature, whereas well-inhibited 15%hydrochloric acid solution showed a rate of corrosion of 1020 mild steelof about 7 mils per year at room temperature. At a temperature of about71° C. a well-inhibited 15% hydrochloric acid solution showed a rate ofcorrosion of 1020 mild steel of about 40 mils per year. However, mosthydrochloric acid corrosion inhibitors are known to breakdown quitequickly, frequently at about 12 hours at bottomhole conditions. Incontrast, the representative acidic aqueous solution of the inventionshowed a rate of corrosion of 1020 mild steel of 40 mils per year overan extended test at about 82° C.

Representative acidic aqueous solutions of the invention display,against calcium carbonate, a spending time at room temperatureapproximately 40 times slower than that of a conventional hydrochloricacid-based acidizing fluid, and a spending time of about 93° C.approximately 20 times slower than a conventional hydrochloricacid-based acidizing fluid. The retarded reaction rate is beneficial inhigh temperature formations in which conventional acidizing fluids spendtoo rapidly for effective stimulation.

The primary constituents of the acidic aqueous solutions are as follows:

PHOSPHORIC ACID

The phosphoric acid employed in the acidic aqueous solutions istypically a technical grade, since greater purity is not normallyrequired. The phosphoric acid employed is conventionally a 54%, 75%, or85% technical grade and the choice of concentration can be varieddependent upon cost or, in the case of concentrate manufacture, on thedesired acid concentration of the concentrate.

CITRIC-ACID-MONOETHANOLAMINE ELIMINATION REACTION PRODUCT

The citric acid-monoethanolamine elimination reaction product can beprepared according to the general teachings of Schwartz U.S. Pat. No.3,095,379, issued June 25, 1963. Contrary to the teachings of Schwartz,it has however been found that it is neither necessary that the citricacidmonoethanolamine elimination reaction be conducted at a temperatureof at least about 150° C., or that a temperature of at least about 150°C. be maintained until gas evolution ceases. The elimination reactiondoes in fact proceed at temperatures less than 150° C., although thereaction rate is temperature dependent. Furthermore, the eliminationreaction yields active product as of the initial exothermicity and thecessation of gas evolution merely indicates that the eliminationreaction has gone to completion. For purposes of the presentspecification, any reference to a particular weight % of the citricacid-monoethanolamine elimination reaction product is thus intended torefer to that particular weight % of active elimination reactionproduct. Schwartz is however correct in stating that the reaction mustbe carried out at a temperature less than the decomposition temperature.

The elimination reaction product is best characterized as a highly polarsurface active agent which is a citric acid-monoethanolamine eliminationreaction product characterized by a visible absorption spectrum whichdisplays an increasing extinction coefficient from the red to the nearU.V. with some structure, an absorption maximum at 475 mn, a plateaufrom 460 mn to 465 nm, an absorption maximum at 450 nm and extinctioncoefficients as follows:

ε₄₇₅ =0.325 l/gm.cm

ε₄₆₂ =0.379 l/gm.cm

ε₄₅₀ =0.509 l/gm.cm.

FERROUS METAL CORROSION INHIBITOR

Preferred corrision inhibitors are organic passivators such as diethylthiourea (Pennzone E®), dibutyl thiourea, O.B.Hibit®, Nambit®, NACAP®,and the like. The corrosion inhibitor can also be of the filming amineor filming amine/propargyl alcohol types but these are likely to displayrelatively fast inhibition breakdown at bottomhole temperatures.

Diethyl thiourea is particularly preferred as the corrosion inhibitordue to its solubility and thermal stability at low pH.

Dependent upon the particular corrosion inhibitor employed, the amountused is likely to vary from about 0.1 to about 1 weight %.

For the preferred corrosion inhibitor, diethyl thiourea, the amount usedis in the range of from about 0.1 to about 1 weight %, the preferredrange being from about 0.2 to about 0.5 weight %, with the particularlypreferred amount being about 0.25 weight %.

Other suitable ferrous metal corrosion inhibitors will be obvious tothose skilled in the art.

While the acidic aqueous solutions are multi-functional, the amounts ofphosphoric acid and citric acid-monoethanolamine elimination reactionproduct in the acidic aqueous solutions can be varied, within the broadranges of from about 5 to about 36 weight % phosphoric acid and fromabout 0.6 to about 5 weight % citric acid-monoethanolamine eliminationreaction product, dependent upon the primary function desired of aparticular treatment.

When the primary desired function of a treatment is the dissolution ofacid soluble deposits, such as carbonates, many sulfides and manyoxides, the preferred ranges are from about 15 to about 22 weight %phosphoric acid and from about 1 to about 1.5 weight % citricacid-monoethanolamine elimination reaction product, with theparticularly preferred amounts being about 20 weight % phosphoric acidand about 1.3 weight % citric acid-monoethanolamine elimination reactionproduct.

When the primary desired function of a treatment is to dehydrate and/orcontrol the hydration of rehydration of clays, in other words when theprimary function is clay shrinking and/or stabilizing, the preferredranges are from about 5 to about 22 weight % phosphoric acid and fromabout 1 to about 2.5 weight % citric acid-monoethanolamine eliminationreaction product, with the particularly preferred amounts again beingabout 20 weight % phosphoric acid and about 1.3 weight % citricacid-monoethanolamine elimination reaction product.

When the primary desired function of a treatment is the penetration anddispersion of acid insoluble organic and inorganic deposits, includingoils, waxes, asphaltenes, resin precipitates of petroleum origin, mudsand clays, the preferred amounts are from about 10 to about 28 weight %phosphoric acid and from about 1 to about 5 weight % citricacid-monoethanolamine elimination reaction product, with theparticularly preferred amounts being about 24 weight % phosphoric acidand about 2.5 weight % citric acid-monoethanolamine elimination reactionproduct.

Other constituents which can be incorporated in the acidic aqueoussolutions are as follows:

PHOSPHATE CRYSTAL MODIFYING AGENT

A phosphate crystal modifying agent is included in the acidic aqueoussolution when it is to be used for treating a well completed in alimestone or dolomitic formation or a sandstone formation having asubstantial carbonate content, i.e. of about 4% or more. The phosphatecrystal modifying agent should be incorporated in the acidic aqueoussolution in an amount effective to control crystal growth, minimizecrystal agglomeration and minimize crystal adhesion to the formation.

Suitable phosphate crystal modifying agents include water soluble low tomedium molecular weight polyacrylate polymers such as Calnox 214® (arelatively low molecular weight polymer which innhibits carbonate andsulfate scale). While phosphonates, phosphonic acids andpolyphosphonates are not in themselves effective as phosphate crystalmodifying agents, approximately equal combinations of a low to mediummolecular weight polyacrylate polymer together with a member of thegroup consisting of water soluble phosphonates, phosphonic acids andpolyphosphonates have proven most effective. Examples of this type ofphosphate crystal modifying agent are Calnox 167® (a blend of an organicphosphonate and a low molecular weight polymer), Arco Base 1220® (aneutralized blend of a phosphonic acid and an acid form of a lowmolecular weight polymer) and CASI 773® (a combination of a polymer anda phosphonate).

Particularly preferred is CASI 773® due to its solubility and thermalstability at low pH.

Dependent upon the particular phosphate crystal modifying agentemployed, the amount used is likely to vary from about 0.2 weight % upto the solubility limit.

For the preferred phosphate crystal modifying agent, CASI 773®, theamount used is in the range of about 0.2 to about 2 weight %, thepreferred range being from about 0.5 to about 1 weight %, with theparticularly preferred amount being about 0.8 weight %.

Other phosphate crystal modifying agents will be obvious to thoseskilled in the art.

SURFACTANT

While the citric acid-monoethanolamine elimination reaction product is asurface active agent, it is frequently desirable to include an amount ofanother surfactant sufficient to impart improved surface activeproperties. Many nonionic or anionic surfactants are useful for thispurpose. Alkyl aryl sulfonates are preferred, in particular Dowfax 2Al®(a mixture of sodium monododecylphenoxybenzenedisulfonate and sodiumdidodecylphenoxybenzenedisulfonate).

For the preferred additional surfactant, Dowfax 2Al®, the amount used isin the range of about 0.05 to about 3 weight %, the preferred rangebeing from about 0.2 to about 1 weight %, with the particularlypreferred amount being about 0.5 weight %.

Other suitable nonionic or anionic surfactants will be obvious to thoseskilled in art.

HYDROXYACETIC ACID

Hydroxyacetic acid can be included in the acidic aqueous solutions ofthe invention in an amount in the range of from about 0.05 to about 1.5weight %, the preferred range being from about 0.2 to about 0.6 weight%, with the particularly preferred amount being about 0.3 weight %.

While the exact function of the hydroxyacetic acid is not known it isspeculated, without wishing to be bound by theory, that thehydroxyacetic acid acts either as a coupling agent and/or enhances thepolarity of the citric acidmonoethanolamine elimination reaction productand/or acts as a retardant to keep the pH of largely expended acidicaqueous solutions low enough to minimize precipitation of calcium andmagnesium phosphates, carbonates and sulfates.

ACID STABLE ANTIFOAMING AGENT

In some applications requiring large volumes of the acidic aqueoussolutions of the invention a tendency to foam may cause handlingproblems during transfer from vessel to vessel.

It has been found that at least several acid stable antifoaming agents,including Antifoam B® and NOSI®, control or eliminate the foamingtendency. The preferred acid stable antifoaming agent is NOSI® whichdisplays antifoaming activity at a concentration of as little as about20 ppm, and effectively renders the acidic aqueous solutions nonfoamingat a concentration of about 100 ppm.

Other suitable acid stable antifoaming agents will be obvious to thoseskilled in the art.

Concentrated acidic aqueous solutions suitable for dilution at the siteof use to the desired acid concentration, to avoid undueshipping/freight charges between the manufacturing plant and site ofuse, can be prepared having a phosphoric acid concentration of up toabout 85 weight %, which represents the most concentrated technicalgrade presently commercially available. The amounts of the citricacid-monoethanolamine elimination reaction product and the ferrous metalcorrosion metal inhibitor, as well as the amounts of any of theadditional constituents such as the phosphate crystal modifying agent,additional surfactant, hydroxyacetic acid and acid stable antifoamingagent, can be varied in proportion to the acid concentration of theconcentrate and the ultimate dilution intended.

The following examples are representative of laboratory scalepreparation and testing of compositions within the purview of theinvention:

EXAMPLE 1

61 gm of monoethanolamine and 96 gm of citric acid are added to a beakerwhich is then heated to and maintained at 150° C. until gas evolutionceases. The resultant citric acid-monoethanolamine elimination reactionproduct is permitted to cool to below 100° C. and then dissolved in 300ml of 75% phosphoric acid. The resultant solution is added to a further1400 ml of 75% phosphoric acid and stirred until homogeneous. 20 gm ofdiethyl thiourea is subsequently added and the resultant solution againstirred until homogeneous.

The resultant solution is representative of a concentrate particularlyintended for ultimate use in wells completed in substantiallycarbonate-free sandstone formations, and which can be readily dilutedwith water to the desired acid concentration.

EXAMPLE 2

The procedure of Example 1 is repeated and subsequently 65 gm of CASI773® is added and the resultant solution stirred until homogeneous.

The resultant solution is representative of a concentrate particularlyintended for ultimate use in wells completed in limestone or dolomiticformations or sandstone formations having a substantial carbonatecontent, and which can be readily diluted with water to the desired acidconcentration.

EXAMPLE 3

When it is desired to enhance the surface active properties of theconcentrate the procedures of Examples 1 and 2 are repeated and to eachof the resultant solutions is added 34 gm of Dowfax 2Al® and theresultant solutions stirred until homogeneous.

EXAMPLE 4

When it is desired that hydroxyacetic acid be included in theconcentrates, the procedures of Examples 1, 2 and 3 are repeated and toeach of the resultant solutions is added 28 gm of hydroxyacetic acid andthe resultant solutions stirred until homogeneous.

EXAMPLE 5

20 ml of an acidic aqueous solution comprising about 5 weight %phosphoric acid, about 1.3 weight % citric acid-monoethanolamineelimination reaction product and about 0.1 weight % diethyl thiourea wasadded to a 20 ml sample of hydrated Wyoming bentonite, a typicaldrilling mud. 20 ml of 5 weight % hydrochloric acid was added to another20 ml sample of hydrated Wyoming bentonite, and 20 ml of water added toa further 20 ml sample of hydrated Wyoming bentonite. After exposure,the volume of the Wyoming bentonite treated with the acidic aqueoussolution according to the invention had decreased through dehydration byabout 20% compared to that of the water treated control sample, whereasthe volume of the Wyoming bentonite treated with the hydrochloric acidsolution had only decreased by about 10% compared to that of the watertreated control sample. The supernatant from both acid treated sampleswas subsequently decanted and 20 ml of distilled water added to eachsample. After exposure, the Wyoming bentonite which had been treatedwith the acidic aqueous solution according to the invention had nothydrated, at least to any appreciable extent, whereas the Wyomingbentonite which has been treated with the hydrochloric acid had hydratedback to approximately its original volume. The example thus demonstratesthe effectiveness of acidic aqueous solutions according to the inventionfor the purpose of shrinking and stabilizing clays.

EXAMPLE 6

20 ml each of acidic aqueous solutions comprising:

(a) about 20 weight % phosphoric acid, about 0.1 weight % citricacid-monoethanolamine elimination reaction product and about 0.3 weight% diethyl thiourea;

(b) about 20 weight % phosphoric acid, about 0.5 weight % citricacid-monoethanolamine elimination reaction product and about 0.3 weight% diethyl thiourea;

(c) about 20 weight % phosphoric acid, about 1 weight % citricacid-monoethanolamine elimination reaction product and about 0.3 weight% diethyl thiourea;

(d) about 20 weight % phosphoric acid, about 2.5 weight % citricacid-monoethanolamine elimination reaction product and about 0.3 weight% diethyl thiourea; and

(e) about 20 weight % phosphoric acid, about 5 weight % citricacid-monoethanolamine elimination reaction product and about 0.3 weight% diethyl thiourea

were added to 20 ml samples of hydrated Wyoming bentonite. Uponexposure, the volume of the Wyoming bentonite of each sample haddecreased through dehydration compared to that of a corresponding watertreated control sample. The volume decrease of the sample treated withsolution (a) was however very minimal. Additionally, in the sampletreated with solution (e), the clay plug was not dispersed. Thesupernatant from each of the acid treated samples was subsequentlydecanted and 20 ml of distilled water added to each. After exposure, theWyoming bentonite samples which had been treated with solutions (a) and(b) increased in volume to a far greater degree than the other samples.The results indicate, at least with the relative amounts of clay/acidicaqueous solution tested, that a given minimum of the citricacid-monoethanolamine elimination reaction product is necessary in theacidic aqueous solutions to dehydrate clays and to prevent clays fromhydrating, whereas at an amount of about 5 weight % the citricacid-monoethanolamine elimination reaction product in the acidic aqueoussolutions appears to interfere with clay dispersion.

EXAMPLE 7

A core of shaley sandstone from the Viking formation in the HamiltonLake field, Alberta, Canada, where wells characteristically displaywater sensitivity, was tested by Core Labs - Canada Ltd., Calgary,Alberta, Canada, in the following manner:

The core was artificially cleaned and dried, and displayed an initialpermeability of about 366 milliDarcy's to a test brine solution under adifferential pressure of about 10 psi. As is typical of this type ofcore, permeability had decreased to about 133 milliDarcy's duringpassage of 30 pore volumes of the brine, i.e. during approximatelyone-half hour of contact time to the brine. 30 pore volumes of an acidicaqueous solution comprising about 20 weight % phosphoric acid, about 1.3weight % citric acid-monoethanolamine elimination reaction product,about 0.25 weight % diethyl thiourea, about 0.5 weight % Dowfax 2Al® andabout 0.3 weight % hydroxyacetic acid were then passed through the core,again under a differential pressure of about 10 psi. No suspended solidswere found in the effluent. Subsequent permeability of the core to thesame test brine was stable at about 321 milliDarcy's after extendedcontact. The fact that there were no suspended solids in the acidicaqueous solution effluent indicates that the increase in permeability isdue to dehydration (shrinkage) of water sensitive clays rather thandislodgement of clay particles from the core. Furthermore, the stablepermeability displayed to the test brine following treatment with theacidic aqueous solution indicates that the acidic aqueous solutionminimizes the hydration of dehydrated clays.

EXAMPLE 8

A limestone core from the Nisku formation in the Pembina field, Alberta,Canada, was tested by Core Labs - Canada Ltd., Calgary, Alberta, Canada,in the following manner:

The core was artificially cleaned and dried, and displayed an initialpermeability of about 1.5 milliDarcy's to 30 pore volumes of a testbrine solution under a differential pressure of about 10 psi. By thetime 5 pore volumes of an acidic aqueous solution comprising about 20weight % phosphoric acid, about 1.3 weight % citricacid-monoethanolamine elimination reaction product, about 0.25 weight %diethyl thiourea, about 0.5 weight % Dowfax 2Al® and about 0.3 weight %hydroxyacetic acid had been passed through the core, again under adifferential pressure of about 10 psi, the permeability had decreased toabout 0.06 milliDarcy's. In other words, the core had essentiallyblocked. The acidic aqueous solution effluent was found to containsignificant amounts of sediment which proved to be calcium phosphate.

EXAMPLE 9

A limestone core from the Leduc formation in the Leduc field, Alberta,Canada, was tested by Core Labs - Canada Ltd., Calgary, Alberta, Canada,in the following manner:

The core was artificially cleaned and dried, and displayed an initialpermeability of about 0.28 milliDarcy's to 30 pore volumes of a testbrine solution under a differential pressure of about 10 psi. Afterabout 8 pore volumes of an acidic aqueous solution comprising about 20weight % phosphoric acid, about 1.3 weight % citricacid-monoethanolamine elimination reaction product, about 0.25 weight %diethyl thiourea, about 0.8 weight % CASI 773®, about 0.5 weight %Dowfax 2Al® and about 0.3 weight % hydroxyacetic acid had been passedthrough the core, again under a differential pressure of about 10 psi,there was a breakthrough and before the test apparatus could beshut-down about 500 pore volumes of the acidic aqueous solution hadpassed through the core. The core displayed significant deteriorationand the acidic aqueous solution effluent initially had some solid matterparticles, presumed to be calcium carbonate, which subsequentlydissolved. Subsequent permeability of the core to the same test brine,again at a differential pressure of about 10 psi, was about 25.5milliDarcy's. The results thus indicate a very significant increase inpermeability as a result of the treatment with the acidic aqueoussolution of the invention. Furthermore, as compared with the results ofExample 8, this example demonstrates the desirability, or evennecessity, of incorporating a phosphate crystal modifying agent, in thisinstance CASI 773®, in acidic aqueous solutions according to theinvention which are to be used for treating limestone or dolomiticformations or sandstone formations having a substantial carbonatecontent.

Representative composition concentrates within the purview of theinvention can be prepared on a commercial scale as follows:

EXAMPLE 10

460 pounds of monoethanolamine and 700 pounds of citric acid are addedto a reaction vessel. After the initial exothermic reaction subsides thevessel is heated to and maintained at 150° C. until gas evolutionceases, typically about 6 hours. The resultant citricacid-monoethanolamine elimination reaction product is permitted to coolto below 100° C., 1500 U.S. gallons of 75% phosphoric acid added and theresultant solution stirred until homogeneous. To the solution issubsequently added 334 pounds of diethyl thiourea, 576 pounds ofhydroxyacetic acid and 719 pounds of Dowfax 2Al® and the resultantsolution again stirred until homogeneous.

This concentrate is particularly intended for ultimate use in wellscompleted in substantially carbonate-free sandstone formations, and canbe readily diluted with water to the desired acid concentration.

EXAMPLE 11

A concentrate for ultimate use in wells completed in limestone ordolomitic formations or sandstone formations having a substantialcarbonate content can be prepared by repeating the procedure of Example10 and additionally adding 1150 pounds of CASI-773® and stirring untilhomogeneous. Again the concentrate can be readily diluted with water tothe desired acid concentration.

An acid stable antifoaming agent can also be added to the concentratesof Examples 10 and 11, if desired.

The following examples are representative of field use of compositionswithin the purview of the invention:

EXAMPLE 12

495 US gallons of an acidic aqueous solution comprising about 36 weight% phosphoric acid, 2.4 weight % citric acidmonoethanolamine eliminationreaction product, about 0.4 weight % diethyl thiourea, about 0.7 weight% Dowfax 2Al® and about 0.6 weight % hydroxyacetic acid were injectedinto a well completed in a substantially carbonate-free sandstoneformation in the Gilbey field, Alberta, Canada. The solution wasinjected at a pressure greater than the formation pressure but below thefracturing pressure, in particular at a wellhead pressure of about 1500psi, in a situation where there was approximately 2800 psi ofhydrostatic head, since a fast feed rate was believed desirable. Thewell was making no fluids prior to treatment and, in fact, after beingshut-in for over two weeks developed only 40 psi pressure on the casing.After injection the casing pressure climbed to 70 psi in less than 24hours, indicating a significantly improved formation permeability. Theinjection was performed as a pre-fracturing clean-up.

EXAMPLE 13

A water injection well completed in a substantially carbonate-freesandstone formation in the Ferrier field, Alberta, Canada, had forseveral years accepted only about 50 barrels of water per day at awellhead pressure of 2200 psi. Prior hydrochloric acid/additivetreatments had not significantly altered the water acceptance of theformation. Treatment of the well with 1200 US gallons of an acidicaqueous solution comprising about 8 weight % phosphoric acid, about 0.6weight % citric acid-monoethanolamine elimination reaction product,about 0.15 weight % diethyl thiourea, about 0.2 weight % Dowfax 2Al® andabout 0.2 weight % hydroxyacetic acid, under the same 2200 psi wellheadpressure, improved the formation permeability to the point of acceptingabout 128 barrels per day of water at a wellhead pressure of 2200 psi.The increased water acceptance is indicative of a significant increasein formation permeability as a result of the treatment.

EXAMPLE 14

A well completed in a substantially carbonate-free sandstone formationin the West Pembina field, Alberta, Canada, and producing from the BellyRiver formation failed to respond to at least one prior conventionalHF/HCl mud acid treatment. Little if any gas, and no oil, was beingproduced by the well. 500 US gallons of an acidic aqueous solutioncomprising about 20 weight % phosphoric acid, about 1.3 weight % citricacid-monoethanolamine elimination reaction product, about 0.25 weight %diethyl thiourea, about 0.5 weight % Dowfax 2Al® and about 0.3 weight %hydroxyacetic acid were injected into the formation at a wellheadpressure of about 2200 psi. This injection pressure was significantlyless than the injection pressure of about 3100 psi necessary for theprevious HF/HCl mud acid treatment. Following treatment the well wasflowing significantly more gas. Fully comparative figures are nothowever available since the well was newly completed and thus noproduction figures had been generated. However, the fact that thecomposition according to the invention could be injected at asignificantly lower pressure than the previous HF/HCl mud acidtreatment, and the relative increase in gas flow, indicate significantimprovement in formation permeability.

EXAMPLE 15

A well completed in vugular limestone in the Redwater field, Alberta,Canada, was producing about 13 barrels of oil per day in a field wheregreater production was typical. Several hydrochloric acid/additivetreatments had been carried out with no significant increase inproduction. 1000 US gallons of an acidic aqueous solution comprisingabout 20 weight % phosphoric acid, about 1.3 weight % citricacid-monoethanolamine elimination reaction product, about 0.25 weight %diethyl thiourea, about 0.8 weight % CASI 773®, about 0.5 weight %Dowfax 2Al® and about 0.3 weight % hydroxyacetic acid were injectedunder a wellhead pressure of about 100 psi, in a situation where therewas a hydrostatic head of approximately 2200 psi. Once the acidicaqueous solution introduced into the formation had been produced out ofthe well, about 20 hours, the well production rate increased to about 78barrels of oil per day.

EXAMPLE 16

A well completed in a dolomitic formation in the Beaverhill Lakeformation in the Swanhills field, Alberta, Canada, had never been alarge producer. On a repetitive cycle of being pumped for 5 days andthen shut-in for 10 days the well was producing about 160 barrels of oilper cycle. The production declined to about 70 barrels of oil per cycle.While the area is known to suffer from asphaltene drop-out and does havescale problems, although this particular well produced only traces ofwater, the reason for the production decline was not known. About 750 USgallons of an acidic aqueous solution comprising about 20 weight %phosphoric acid, about 1.3 weight % citric acid-monoethanolamineelimination reaction product, about 0.25 weight % diethyl thiourea,about 0.8 weight % CASI 773®, about 0.5 weight % Dowfax 2Al® and about0.3 weight % hydroxyacetic acid were displaced into the formation by wayof an uncontrolled squeeze, i.e. by means of the hydrostatic head in theannulus of the well bore, the pressure of which exceeded the formationpressure. The well was subsequently put back on pump and has sinceproduced approximately 200 barrels of oil per cycle. The increase inproduction is indicative of improved formation permeability.

The examples demonstrate that the acidic aqueous solutions possess clayshrinking and stabilizing properties, an ability to dissolve organicdeposits such as carbonates, and an ability to penetrate and disperseinsoluble deposits such as muds and clays. Additionally, while notdemonstrated by the examples, the acidic aqueous solutions have provento be of a non-emulsifying character and, in some cases, have displayedan ability to break water-in-oil emulsions of the type typicallypromoted by hydrochloric acid-based acidizing fluids.

The acidic aqueous solutions of the invention are also suitable for useas perforating fluids.

Cased wells are often perforated in such a condition that thehydrostatic pressure of the column of fluid in the casing overbalancesthe formation pressure with the result that, upon perforation, fluid inthe casing flows into the formation. Ideally the perforating fluidshould not cause any formation damage such a decrease in permeabilityand, in fact, should assist in cleaning the immediate perforations andimprove the formation permeability. The ability of the acidic aqueoussolutions of the invention shrink Wyoming bentonite, a typical drillingmud, as is shown in Example 5, the ability to dissolve acid solubledeposits, as in shown in Example 9, and the relative lack of secondaryprecipitation are all desired characteristics of a perforating fluid andthus indicative of the suitability of acidic aqueous solutions of theinvention as perforating fluids.

The acidic aqueous solutions of the invention are also suitable for useas formation fracturing fluids.

The acidic aqueous solutions of the invention are compatible withconventional gelling agents, such as guar flour, and conventionalpropping agents, such as graded sand of uniform spherical granularconfiguration, typically a 20 to 40 mesh silica sand, which are commonlyemployed to retain a formation in a fractured condition.

Modifications and variations within the true broad spirit and scope ofthe invention will be apparent to those skilled in the art.

We claim:
 1. A method of treating a subterranean limestone or dolomiticformation or a sandstone formation having a substantial carbonatecontent associated with a well completed in said limestone or dolomiticformation or said sandstone formation having a substantial carbonatecontent comprising displacing into said formation through said well aneffective amount of an acidic aqueous solution comprising from about 5to about 36% by weight phosphoric acid, from about 0.6 to about 5 weight% of a highly polar surface active agent which is a citricacid-monoethanolamine elimination reaction product characterized by avisible absorption spectrum which displays an increasing extinctioncoefficient from the red to the near U.V. with some structure, anabsorption maximum at 475 nm, a plateau from 460 nm to 465 nm, anabsorption maximum at 450 nm, and extinction coefficients asfollows:ε₄₇₅ =0.325 l/gm.cm ε₄₆₂ =0.379 l/gm.cm ε₄₅₀ =0.509 l/gm.cm,aneffective amount of a ferrous metal corrosion inhibitor, and an amountof a phosphate crystal modifying agent sufficient to control crystalgrowth, minimize crystal agglomeration and minimize crystal adhesion tothe formation, said phosphate crystal modifying agent being selectedfrom the group consisting of polyacrylate polymers and blends of suchpolymers with a phosphonic acid or a phosphonate.
 2. A method accordingto claim 1 wherein the acidic aqueous solution is displaced into saidformation under a pressure greater than the formation pressure but lessthan that causing fracture of the formation.
 3. A method according toclaim 2 wherein in the acidic aqueous solution the phosphoric acid ispresent in an amount of from about 15 to about 22 weight % and thecitric acid-monoethanolamine elimination reaction product is present inan amount of from about 1 to 1.5 weight %.
 4. A method according toclaim 2 wherein in the acidic aqueous solution the phosphoric acid ispresent in an amount of from about 5 to about 22 weight % and the citricacid-monoethanolamine elimination reaction product is present in anamount of from about 1 to about 2.5 weight %.
 5. A method according toclaim 2 wherein in the acidic aqueous solution the phosphoric acid ispresent in an amount of from about 10 to about 28 weight % and thecitric acid-monoethanolamine elimination reaction product is present inan amount of from about 1 to about 5 weight %.
 6. A method according toclaims 3, 4, or 5 wherein in the acidic aqueous solution there isadditionally included an amount of a nonionic or anionic surfactantsufficient to impart improved surface active properties andhydroxyacetic acid.
 7. A method according to claims 3, 4 or 5 wherein inthe acidic aqueous solution the ferrous metal corrosion inhibitor is adialkyl thiourea, and there is additionally included an amount of anonionic or anionic surfactant sufficient to impart improved surfaceactive properties and hydroxyacetic acid.
 8. A method according toclaims 3, 4 or 5 wherein in the acidic aqueous solution the ferrousmetal corrosion inhibitor is a dialkyl thiourea, and solution there isadditionally included an amount of an nonionic or anionic surfactantsufficient to impart improved surface active properties, hydroxyaceticacid and an amount of an acid stable antifoaming agent sufficient toprevent foaming.
 9. A method according to claim 1 wherein the acidicaqueous solution is displaced into said formation under a pressuresufficient to fracture the formation.
 10. A method according to claim 9wherein in the acidic aqueous solution the phosphoric acid is present inan amount of from about 15 to about 22 weight % and the citricacid-monoethanolamine elimination reaction product is present in anamount of from about 1 to about 1.5 weight %.
 11. A method according toclaim 9 wherein in the acidic aqueous solution the phosphoric acid ispresent in an amount of from about 5 to about 22 weight % and the citricacid-monoethanolamine elimination reaction product is present in anamount of from about 1 to about 2.5 weight %.
 12. A method according toclaim 9 wherein in the acidic aqueous solution the phosphoric acid ispresent in an amount of from about 10 to about 28 weight % and thecitric acid-monoethanolamine elimination reaction product is present inan amount of from about 1 to about 5 weight %.
 13. A method according toclaims 10, 11 or 12 wherein in the acidic aqueous solution there isadditionally included an amount of a nonionic or anionic surfactantsufficient to impart improved surface active properties andhydroxyacetic acid.
 14. A method according to claims 10, 11 or 12wherein in the acidic aqueous solution the ferrous metal corrosioninhibitor is a dialkyl thiourea, and there is additionally included anamount of a nonionic or anionic surfactant sufficient to impart improvedsurface active properties and hydroxyacetic acid.
 15. A method accordingto claims 10, 11 or 12 wherein in the acidic aqueous solution theferrous metal corrosion inhibitor is a dialkyl thiourea, and there isadditionally included an amount of a nonionic or anionic surfactantsufficient to impart improved surface active properties, hydroxyaceticacid and an amount of an acid stable antifoaming agent sufficient toprevent foaming.